Earth loop resistance lowering

[and1145]

I have just renewed my fuse type consumer unit and have fitted the rcd/ mcb type, I have a TT connection and could not find an earth rod anywhere on the premises, so having now fitted one I am getting a reading of 21 ohms when carrying out an earth loop impedance test. The recommended value for TT suplies is 20 ohms (i believe).

The question I want to ask is how is the best way of lowering the impedance ?

Would another earth spike improve it?

The house was re-wired around 10 years ago.

 

[Lectrician]

200

 

[ban-all-sheds]

R x RCD tripping current <= 50

??

 

[fireman22]

NICEIC state 100 ohms as value they say is minimum you should allow

 

[ban-all-sheds]

Oh well, they know best, don't they.

 

[Lectrician]

NIC tell us 200 max.

 

[and1145]

So giving the reading of 21 ohms, It should be fine ?
It is fed from a 3 phase TT supply in one of the outbuildings and has an old RCD (not sure what rating, type s on the front) then comes across into the house via pvc 16mm twin into the new consumer unit with a 30mA RCD and has never given any trouble.
Appologies but new to the TT system.

Andy

 

[fireman22]

lectrician, new snags and solution book states 100 ohms?????

 

[fireman22]

maximum value that is.

 

[ban-all-sheds]

OSG says 21 ohms...

 

[Lectrician]

BS7430 does suggest an impedance higher than 100ohm may be unstable, although this is in relation to a stake for a genset.

I will look in the nic tech manual when I have time.

 

[mapj1]

Depends on the RCD - with a 100 or even 300ma type common on farm incomers 200 ohms is approaching the upper limit. If its all 30mA RCD, or even 10mA RCD as seen in some 'tent city' installations, then a higher resistance is OK - it MUST be low enough trip the RCD, even in dry weather, without raising the victim metalwork above 50V (or 25V on the farm). So 50V times 30mA is about 1500 Ohms, or 750 ohms on the farm..
But, to err on the safe side, as low as poss is better. If more than say 100ohms on a wet day, then hammer in another spike in parallel 1 rod length away or more... it may well be worse in summer, and much more than 200 ohms and I'd be suspecting either its driven into in dry gravel, or the end of the rod has rotted off just below the surface, and in either of those cases needs a bit of help.
The 21 ohms is the highest impedance you can assume for the REC transformer end, if measuring by loop tester unless they tell you otherwise when you ring up. In practice safer to assume all the measured resistance is at your end.
hope this makes it clearer where the numbers come from.

 

[and1145]

Depends on the RCD - with a 100 or even 300ma type common on farm incomers 200 ohms is approaching the upper limit. If its all 30mA RCD, or even 10mA RCD as seen in some 'tent city' installations, then a higher resistance is OK - it MUST be low enough trip the RCD, even in dry weather, without raising the victim metalwork above 50V (or 25V on the farm). So 50V times 30mA is about 1500 Ohms, or 750 ohms on the farm..
But, to err on the safe side, as low as poss is better. If more than say 100ohms on a wet day, then hammer in another spike in parallel 1 rod length away or more... it may well be worse in summer, and much more than 200 ohms and I'd be suspecting either its driven into in dry gravel, or the end of the rod has rotted off just below the surface, and in either of those cases needs a bit of help.
The 21 ohms is the highest impedance you can assume for the REC transformer end, if measuring by loop tester unless they tell you otherwise when you ring up. In practice safer to assume all the measured resistance is at your end.
hope this makes it clearer where the numbers come from.

Hi,

Thanks for your reply, a little clearer now but with my TT supply there is no earth coming from the rec's transformer. All I get is the 3 phases coming in and one phase is tapped into, to supply the main house. The 21 ohms is purely what the earth rod is giving on an impedance loop test. I just wanted to clear this up before I call the building control to have the installation checked. Everyone I ask says the old 1 ohm rule and aren't sure about TT supplies.

Andy

 

[plugwash]

TT will be FAR higher than a TN system thats why you have the main rcd.

your figure seems to be very good for a TT though not something to worry about.

 

[Lectrician]

The transformers rod will play part of your EFL on a TT system.